Modern processing plants employ elaborate control systems for controlling a vast array of process control field devices and other process related equipment. Process control systems, like those used in chemical, petroleum or other processes, typically include one or more centralized or decentralized process controllers communicatively coupled to at least one host or operator workstation and to one or more process control and instrumentation devices such as, for example, field devices, via analog, digital or combined analog/digital buses. Field devices, which may be, for example, valves, valve positioners, switches, transmitters, and sensors (e.g., temperature, pressure, and flow rate sensors), are located within the process plant environment, and perform functions within the process such as opening or closing valves, measuring process parameters, increasing or decreasing fluid flow, etc. Smart field devices such as field devices conforming to the well-known FOUNDATION™ Fieldbus (hereinafter “Fieldbus”) protocol or the HART® protocol may also perform control calculations, alarming functions, and other control functions commonly implemented within the process controller.
The process controllers, which are typically located within the process plant environment, receive signals indicative of process measurements or process variables made by or associated with the field devices and/or other information pertaining to the field devices, and execute controller applications. The controller applications implement, for example, different control modules that make process control decisions, generate control signals based on the received information, and coordinate with the control modules or function blocks being performed in the field devices such as HART and Fieldbus field devices. The control modules in the process controllers send the control signals over the communication lines or signal paths to the field devices, to thereby control the operation of the process.
Information from the field devices and the process controllers is typically made available to one or more other hardware devices such as, for example, operator workstations, maintenance workstations, personal computers, handheld devices, data historians, report generators, centralized databases, etc. to enable an operator or a maintenance person to perform desired functions with respect to the process such as, for example, changing settings of the process control routine, modifying the operation of the control modules within the process controllers or the smart field devices, viewing the current state of the process or of particular devices within the process plant, viewing alarms generated by field devices and process controllers, simulating the operation of the process for the purpose of training personnel or testing the process control software, diagnosing problems or hardware failures within the process plant, etc.
While a typical process plant has many process control and instrumentation devices such as valves, transmitters, sensors, etc. connected to one or more process controllers, there are many other supporting devices that are also necessary for or related to process operation. These additional devices include, for example, power supply equipment, power generation and distribution equipment, rotating equipment such as turbines, motors, etc., which are located at numerous places in a typical plant. While this additional equipment does not necessarily create or use process variables and, in many instances, is not controlled or even coupled to a process controller for the purpose of affecting the process operation, this equipment is nevertheless important to, and ultimately necessary for proper operation of the process.
In modern process control systems there is often a need for an operator, an application, a control routine or sub-routine or some other process control entity or control logic to acquire exclusive ownership (i.e., control) over some other process control entity in order to perform a particular process related task. Once a process control entity has been acquired by some other process control entity the acquired process control entity may only be controlled by or respond to the acquiring process control entity. Once the particular task is complete, the acquiring process control entity may release the acquired process control entity, whereupon the acquired process control entity may be acquired by a different process control entity, or is at least available to be controlled by or respond to other process control entities.
In a batch processing environment, for example, a batch manager application may be executing a batch process that calls for a number of unit operations to be performed on a particular process unit. A number of process control field devices may be associated with the processing unit for controlling various processing steps associated with the operation of the processing unit. A particular process control device, control module or other logical entity associated with the processing unit may need to acquire exclusive ownership over come or all of the ancillary field devices associated with the processing unit before the batch unit operations can begin on the processing unit. Once the process control device, control module or other logical unit has acquired the various field devices associated with the processing unit, no other process control devices, control modules or other logical entities can effect the operation of the acquired process control field devices. Once the batch unit operations performed on the processing unit are complete, the process control device, control module or other control logic associated with the processing unit may release the various process control field devices which may then be available to be acquired by other process control devices, control modules or other logical entities associated with other processing units or batch operations.
It is not uncommon for conflicts to arise in a process plant when more than one process control device, control module or other logical entity attempts to acquire the same process control device or other process control entity at the same time. Since ownership of one process control device or process control entity by another process control device or process control entity implies exclusive control over the acquired process control device or process control entity, a decision must be made between competing demands to acquire the same process control device or other process control entity.
A process control system may include an equipment ownership arbitration application for resolving such disputes. Typically an ownership arbitration application will be executed at a centralized location such as a user workstation associated with the process control system. For example, a batch manger application for managing batch processes within a process plant may be implemented in a workstation associated with a process control system. The batch manager application may include an arbitration manager. The computer workstation implementing the batch manager application may be connected to one or more process controllers via a local area network or other communication link. Arbitration requests are provided to the arbitration manager when competing demands for ownership over a single process control device, control module or other logical entity arise. When such conflicts arise, the arbitration manager decides which of the competing demands will be honored and which will be rejected or delayed. The arbitration manager makes arbitration decision based on predefined arbitration rules embedded within the arbitration manager application.
A problem with this arrangement is that the arbitration manager executed on a centralized workstation represents a potential single point of failure that could prevent arbitration requests from being processed. If the workstation fails, or the communications over the LAN fail, arbitration requests may go unanswered. Another problem with existing ownership arbitration systems is that the process entities themselves, those that are acquiring other process control entities as well as those that are being acquired, are not made aware of the arbitration decisions that have been made on their behalf. If the centralized arbitration manager is operating properly, it will prevent a process control entity that has been acquired by some other process control entity from being acquired by yet another process control entity. If the centralized arbitration manager is not operating properly, however, or if communications between the arbitration manager and the acquired process control entity are interrupted, the acquired process control entity itself will be incapable of rejecting the acquisition request of the other process control entity and may improperly be owned by more than one other process control entity at the same time. Finally, a control system may implement multiple workstations implementing multiple batch managers. The individual batch managers may each be executing individual arbitration managers. These independent arbitration managers may not be aware of the arbitration decisions made by the other arbitration managers. An arbitration decision made by one arbitration manager may not be respected (or even considered) by another arbitration manager, which can lead to process control entities being inadvertently acquired by more than one other process control entity at the same time, an untenable situation under most process control circumstances.